1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or the like and, more particularly, to an image forming apparatus including a plurality of developers.
2. Related Background Art
FIG. 11 is a schematic view showing an example of the drive arrangement of a developer selection unit 1 in an image forming apparatus including a plurality of developers.
Note that a photosensitive drum 9 on which an optical image 10 is exposed, and a transfer drum 11 for carrying a recording medium are arranged near the unit 1. The developer selection unit 1 comprises sensor flags 2a, 2b, 2c, and 2d fixed on its rotary surface in correspondence with the developers. A drive gear 5 for transmitting the drive force of a stepping motor 6 is engaged with the rotary surface of the developer selection unit 1.
With this arrangement, a position switching operation of the developer selection unit 1 is performed via the drive gear 5 by driving the stepping motor 6.
A flag sensor 3 for detecting one of the sensor flags 2a, 2b, 2c, and 2d, and a developer/toner sensor 12 for detecting the presence/absence of a developer and the presence/absence of a toner in the developer are arranged as shown. These sensors are connected to a central processing unit (CPU) in a motor control circuit 7. The CPU is connected to a memory 7a and a motor driver 7b, which is further connected to the stepping motor 6.
The selection/positioning operation of a developer is performed by detecting one of the sensor flags 2a, 2b, 2c, and 2d corresponding to developers a, b, c, and d by the flag sensor 3, and by counting the number of driven steps of the stepping motor 6 by the motor control circuit 7.
The selection/positioning operation of a developer will be described below with reference to the timing chart in FIG. 12.
When the developer selection unit 1 is driven, the flag sensor 3 detects the sensor flag 2a of a certain developer (e.g., a) to recognize the position of the developer a. Then, the developer selection unit 1 is driven in a predetermined developing profile (P0-P1-P2-P3), and is stopped at a position where the developer a faces the photosensitive drum 9, thus performing a developing operation. More specifically, when the position of the developer a is recognized (61), the motor control circuit 7 starts the counting operation of the number of driven steps of the stepping motor 6, and accelerates the stepping motor 6 until the count value reaches a predetermined number of steps (62). Then, a predetermined rotational speed, i.e., a drive frequency is maintained until the count value reaches another predetermined number of steps, and thereafter, a predetermined deceleration operation, i.e., a slowdown operation is performed (63) to stop the stepping motor (64).
Each developer has a developing sleeve, a toner, and the like, and is normally as heavy as several hundreds grams to 1 kg or more. Therefore, in order to rotate the developer selection unit which holds the plurality of developers, it is required to control a large inertial load especially upon starting and stopping of the rotation. For this purpose, the time required for each of a startup acceleration operation and a deceleration operation must be long. However, when the developer selection time is long, the throughput of the apparatus is lowered, and an increase in selection time is limited. Therefore, it is required to perform startup and deceleration operations as quick as possible.
In control of a stepping motor, since the motor is normally designed to have a torque margin, a deviation of the stop position of a developer to be controlled need not be taken into consideration. However, in the case of the developer selection unit, since the weight of each developer varies depending on consumption of a toner in the developer, the wear of a sliding portion, and the like, a variation in load balance is very large. For example, as shown in FIG. 13, when the two adjacent developers a and d of the four developers a, b, c, and d are full of respective toners, and the remaining developers b and c are empty, the load on the motor becomes largest.
When developers are detachable from the developer selection unit, four developers for four colors are attached to the developer selection unit in a normal full-color print mode. However, in a monochrome, two-color, or three-color print mode, the four developers are not always attached. For example, as shown in FIG. 14, only two developers 8a and 8b may be attached at adjacent positions. The developer selection unit in this state imposes, to the stepping motor, a load requirement larger by several times than that imposed when the four developers are attached. Since the developers are locally offset, a variation in the load upon rotation is also large.
To say nothing of a normal full-color print mode, when a monochrome print operation or a color print operation of three colors or less is performed for a large number of sheets, since the weights of color toners change during the print process, the weight balance of the developer selection unit with respect to rotation changes. For example, when two adjacent developers of four developers are heavy, and the remaining two developers are light, the load becomes larger than that obtained when all the four developers have uniform weights.
Therefore, even when a single drive profile shown in FIG. 12 is set for the stepping motor, the deceleration operation may become insufficient depending on a change in condition of the developers, and the like. In this case, the stepping motor causes an out-of-phase state, and the developer selection unit 1 does not stop at a required position, e.g., overruns a target stop position by CTaov, as shown in FIG. 13.
When the drive profile of the stepping motor is set to perform a deceleration operation with a margin within a limited time, if the inertia of the developer selection unit 1 is small, considerable vibration and noise are generated in a low-speed range.